Summary of Work: The coccidian Toxoplasma gondii is an intracellular parasite that can infect all mammalian cells. While infections of immunocompetent individuals are usually asymptomatic, toxoplasmosis in immunocompromised individuals can cause serious illness or death. While effective therapies exist for the treatment of toxoplasmosis, they are not without adverse effects, thus establishing a rationale for vaccine therapy. Since immunization with nucleic acids injected intramuscularly can induce both CD4+ and CD8+ T cell-mediated immune responses, and because both types of responses are important in conferring immunity to T. gondii infections, immunization with DNA has the potential to induce protective immunity. To assess the feasibility of achieving immunity to T. gondii we performed vaccinations of BALB/c mice using a cDNA construct that contained the coding sequence for the mature peptide of the P30 surface antigen. Expression was under control of the immediate early promoter of cytomegalovirus. Endpoint dilutions of sera from these animals were compared with controls. Intradermal administration of the plasmid via a biolistic gene gun enhanced these responses several fold. Boosting with a multiple antigenic peptide, which is immunodominant in seropositive animals, enhanced the antibody titers. We have also recently produced recombinant P30 as a fusion protein in CHO cells to aid in the development of enzyme-linked immunosorbent assays and cytotoxic T lymphocyte assays. We are currently evaluating the efficacy of polynucleotide vaccination in mice with different MHC haplotypes.